2014 Volume 20 Issue 5 Pages 997-1004
The physicochemical properties and molecular structure of ginkgo and mung bean starch were investigated. Results showed that ginkgo starch (GBS) prepared in the laboratory had a good purity and displayed smaller granule particle size, higher gelatinization temperatures and lower average molecule weight (Mw) than mung bean starch (MBS). Ginkgo starch noodles (GBSN) and mung bean starch noodles (MBSN) were prepared and the quality evaluation among GBSN, MBSN, commercial mung bean and sweet potato starch noodles (c-MBSN and c-SPSN, respectively) revealed that cooking quality, and partial textural properties of GBSN were close to those of commercial products. The enzymatic digestion analysis of GBSN and MBSN conducted with α-amylase and β-amylase + pullulanase systems suggested that starch noodles were more resistant to β-amylase + pullulanase than α-amylase, and the starch in noodles might be composed of two parts: one part with relatively loose structure could be easily degraded, and the other with compact structure would be degraded more slowly.
Ginkgo (ginkgo biloba L.) is one of the oldest plant species on earth and is considered as a “living fossil” (Nakanishi, 2005). Its fruit consists of an exocarp and a hard shell with the kernel of the seed. The edible kernel contains about 6.4% protein, 2.4% lipid, 35% carbohydrate and some kinds of phytochemicals that have been found to possess health benefits (Miao et al., 2012). In China, the annual harvest of ginkgo seeds is over 12000 tons (Chen et al., 2012), and kernels have traditionally been consumed as a home-style dish after cooking (Miao et al., 2012).
Studies demonstrated that eating raw and cooked ginkgo kernels could cause allergic reaction to humans (Yang et al., 2011), therefore consumption of ginkgo kernels is limited. Ginkgo kernels are used to prepare functional beverage after blending with water and centrifugal separation. In this case, a suitable utilization should be explored for the centrifugal residues. Starch is the most abundant carbohydrate in gingko residues and several researches have been done on the physicochemical properties of ginkgo starch (Miao et al., 2012; Spence and Jane, 1999), but few investigations paid attention to ginkgo starch exploitation. A possible way to use the ginkgo starch in the centrifugal residues might be the preparation of starch noodles which have always been Chinese favorite food and are extensively consumed throughout Asian countries.
Various starches could be used to producing noodles, such as legume starches, tuber or root starches and grain starches, among all of these starch, mung bean starch is regarded as the best raw material (Wang et al., 2010; Tan et al., 2009). The mung bean starch noodles are of good quality which is certainly related to the chemical and physical properties of starch, such as its high amylose content, restricted granule swelling during gelatinization (Lii and Chen, 1981). The amylose could help starch to form gel network structure, and the starch with higher amylose content could form stronger gel structure which could make starch noodles with better quality, such as cooking and textural qualities (Yoenyongbuddhagal and Noomhorm, 2002). The starch with lower degree of swelling has better thermal stability of starch paste, which could produce starch noodles with lower cooking loss (Lii and Chen, 1981). According to recent studies (Miao et al., 2012; Spence and Jane, 1999), the apparent amylose content of ginkgo starch was close to that of mung bean starch. The swelling factors of ginkgo starch are even lower (Hoover et al., 1997). Based on these properties, the centrifugal residues of gingko kernel could be utilized to prepare gingko starch noodles.
The present study focused on the comparative investigation of the physicochemical properties and molecular structure of ginkgo and mung bean starch to verify the technological feasibility of gingko starch noodles. In addition, the understanding of the enzymatic digestion characteristics and the structure of ginkgo starch noodles with mung bean starch noodles as reference was indispensable to undertake more efforts to broaden the industrial application of ginkgo starch and expand the market of ginkgo starch noodles.
Materials Ginkgo seeds were obtained from Xuzhou Tianli biochemical product Co. Ltd., Jiangsu, China. Mung bean starch was purchased from Hengshui starch Co. Ltd., Hebei, China. The c-MBSN and c-SPSN were bought from the local supermarket. Alpha-amylase from Fungus species (EC 3.2.1.1, 60 units/mg solid), β-amylase (E.C 3.2.1.2, 50 units/mg solid), and pullulanase (E.C 3.2.1.41; 30 units/mg solid) were provided by KDN biotech group, Shandong, China.
Starch isolation The ginkgo kernels were shelled manually and homogenized in a JYL-D022 blender (Joyoung Co. Ltd., Shandong, China) with 0.025 mol/L sodium hydroxide solution (1:2, w/v) for 2 min. The slurry was filtrated through the 100 and 200 mesh sieves. The filtrate was allowed to stand for 4 h, and the obtained sediments were mixed with sodium hydroxide solution and stirred gently for 12 h. The mixture was left standing for 4 h and the supernatant was removed. The precipitate was washed three times with distilled water and dried in an oven at 45°C for 14 h. Finally, the starches were finely ground in a high-shear blender for 2 min, and packaged in a valve bag.
Preparation of starch noodles Nine grams of starch were mixed with 45 mL of hot water, and the mixture was gelatinized in an HH-S2 boiling water bath (Jintan medical instrument factory, Jiangsu, China) to obtain starch paste. The starch dough was prepared with starch paste, 86 g of raw starch and 20 mL hot water. The dough was put in a closed vessel with small holes of 1.2 mm diameter at the bottom. The pressure in the vessel was maintained at 0.25 MPa with compressed nitrogen. The dough passed through the holes was cooked in boiling water for 20 s. The obtained starch noodles were cooled in cold water for 1 h, drained and placed evenly on clean porcelain tray. After freezing, the starch noodles were dried in a BZG drum wind drying oven (Shanghai Boxun Industrial Co. Ltd., Shanghai, China) at 45°C for 3 h and stored in plastic bags.
Quality evaluation of starch noodlesCooking quality Starch noodle samples (5 g) were dried at 105°C for 4 h to obtain dry samples (m0). The dry noodles were boiled in 150 mL distilled water for 15 min and drained for 5 min to separate the water, then weighed (m1) immediately. The cooked starch noodles were dried at 105°C again to a constant weight (m2). Cooking loss and swelling index were expressed as:
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Broken rate Starch noodles (50 strands) were boiled in the 500 mL distilled water for 30 min, and drained. The total number X of cooked noodles was recorded. The broken rate (BR) of starch noodles was expressed by the following equation:
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Textural properties Ten strands of starch noodles in the length of 6 cm were boiled in the 250 mL distilled water for 10 min, and then drained. Textural profile analysis was performed by TA. XTplus texture analyzer (Stable Micro System Ltd., UK). A strand of cooked starch noodle was compressed by a cylinder probe (P/35, diameter 35 mm) until the deformation reached to 75% of the diameter of starch noodle (1.7 mm diameter). Pause time between the first and second compression was 3 s, the trigger force was 5 g, and the test speed was 1 mm/s. From the texture profile analysis, hardness, adhesiveness, springiness and cohesiveness were obtained. Seven measurements for each sample were determined with seven strands of starch noodles and the mean value was recorded.
Enzymatic digestion of ginkgo and mung bean starch noodles Starch noodles samples (18 g) were soaked overnight in 250 mL distilled water at 35°C. The mixture was heated in boiling water bath for 1 h, then cooled to room temperature. The mixture was sheared at 9500 rpm for 2 min (Janke & Kunkel, Ika labortechnik) to pass through a 40 mesh sieve. The suspension was transferred quantitatively and diluted to 500 mL with water. Nine thousand units of α-amylase dissolved in 500 mL 0.2 mol/L phosphate buffer (pH 6.0) was added, the sample was maintained at 35°C in a water bath and gently shaken once a day for up to 120 h. Hydrolysate (0.1 mL) was taken every 12 h and diluted to 10 mL volume with distilled water. After centrifugation (5000 rpm, 15 min), the reducing sugar content of supernatant was determined by Somogyi (1952) method. The degree of hydrolysis was defined as:
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The acid hydrolysis was carried out by treating starch noodle with HCl (1 g starch noodles mixed with 20 mL 1 mol/L HCl) at 100°C for 2 h.
Similarly, the digestion was performed using β-amylase + pullulanase in 500 mL 0.01 mol/L acetate buffer (pH 5.3) at 35°C for 60 h. At the end of the digestion, the mixture was independently centrifuged. The supernatant was discarded and the residues were rinsed with water 3 times and then dried in a DZG-6020D vacuum drying oven at 35°C. The dried samples were analyzed by the methods described in the following methods.
Analytical methodsChemical composition of starch Starch, moisture, lipid, and protein contents were determined according to the AACC standard methods (76-13.01, 44-19.01, 58-15.01, 46-09.01, respectively; AACC International, 2010). Amylose contents were determined using a colorimetric method (Miao et al., 2009).
Molecular weight distribution Starch samples (10 mg) were dissolved in 10 mL 50 mmol/L NaNO3/ dimethyl sulfoxide solution (DMSO) solution (through 0.22 µm film). The suspension was heated in a 90°C water bath for 30 min, and then stirred constantly for 24 h at 50°C. The obtained solution was centrifuged at 10000 rpm for 5 min, and the supernatant was filtered through a 0.45 µm nylon syringe filter.
The high performance size-exclusion chromatography system consisted of an HP 1050 series pump (Hewlett-Packard, Valley Forge, PA) and an automatic quantitative valve with a 100 µL injection loop. The detection instrument was composed of a multi-angle laser-light scattering detector (Dawn DSP-F, Wyatt Technology, Santa Barbara, CA) with a He-Ne laser source (λ = 632.8 nm), a K-5 flow cell, and a refractive index detector (model ERC-7512, ERMA Inc., Tokyo, Japan). Two organic SEC columns (Styragel HMW6E and 2, Waters Corporation, Milford, USA) were connected together to determine molecular weight distributions. The column temperature was maintained at 45°C. Fifty mmol/L NaNO3/DMSO solution was the mobile phase and the flow rate was 0.6 mL/min.
T-2000 and T-40 dextran starch was used for normalization of multi-angle photodiode detectors. The data obtained from the MALLS and RI detectors were processed by Astra software (version 5.3.4, Wyatt Technology Corporation, CA, USA). A dn/dc value of 0.07 was used to calculate Mw and average gyration radius (Rz) of starch in DMSO solution.
Differential scanning calorimetry (DSC) The dried GBSN and MBSN were ground by JYL-D022 blender for 2 min to pass through a 100 mesh sieve. The ground sample (3.0 mg) was weighed into aluminum pans (Perkin Elmer Inc., CT, USA). Distilled water (9.0 µL) was added to each pan and hermetically sealed. Sample was allowed to equilibrate at 4°C for 24 h and at room temperature for 1 h prior to analysis, and then scanned at a heating rate of 10°C/min from 30 to 120°C. The DSC was calibrated using indium as a standard and an empty aluminum pan as the reference. The onset temperature (To), peak temperature (Tp), conclusion temperature (Tc) and enthalpy of gelatinization (ΔH) were calculated automatically.
Scanning electron microscopy (SEM) The dried GBSN and MBSN were cut transversely with a sharp blade. The starches, the surfaces and transverse sections of both starch noodles were sputtered with gold and analyzed using an S-4800 SEM (Hitachi Corporation, Tokyo, Japan) at an accelerating voltage of 1 kV.
Chemical composition The chemical composition of starch, which was closely related to the qualities of starch noodles, was given in Table 1. If the moisture content of starch was too high, the starch would not be preseved well that could be a problem in starch process (Wolfgang et al., 1999).The moisture content of ginkgo and mung bean starch ranged between 8.30% and 10.10%, which was far below the maximum value of 20% generally acceptable for noodle production (Yousif et al., 2012),. The protein content was usually regarded as an indicator of purity for legume and cereal starches (Lii and Chen, 1981), and the lipid content of GBS were greatly lower than that of MBS. Amylose could promote starch retrogradation and form helical complex with lipids to give strong gel networks, which make amylose become most important factor influencing starch gel strength (Tan et al., 2009). It was found that the amylose content of GBS (33.90%) was close to that of MBS (34.10%). The chemical compositions obtained indicated that the GBS prepared in laboratory had a good purity compared to MBS and was suitable for noodle preparation.
| Samples | Starch/% | Moisture/% | Crude protein /% | Lipids/% | Amylose/% |
|---|---|---|---|---|---|
| GBS | 89.30 ± 0.23 | 8.30 ± 0.08 | 0.13 ± 0.01 | 0.11 ± 0.02 | 33.90 ± 0.19 |
| MBS | 90.12 ± 0.36 | 10.10 ± 0.12 | 0.32 ± 0.01 | 0.28 ± 0.01 | 34.10 ± 0.23 |
Data are expressed as the mean of three measurements ± SD.
Molecular weight distribution The Mw value could affect swelling, retrogradation and gel properties of starch. The starch with lower Mw is more hydrophilic and more easily retrograded. The Mw and Rz of GBS and MBS were summarized in Table 2. The Mw and Rz of GBS were lower than those of MBS. This result might indicate that the molecules of GBS were smaller than those of MBS, or the proportion of small molecules in GBS was higher.
| Sample | Enzyme | Peak 1 Mw/(kg.kmol−1) | Peak 2 Mw/(kg.kmol−1) | Mw/(kg.kmol−1) | Rz×109/m | ρ×10−27/(kg.kmol−1 · m−3) |
|---|---|---|---|---|---|---|
| GBS | - | - | - | 3.45 (± 0.12) × 107 | 125.75 ± 0.14 | 17.35 ± 0.32 |
| MBS | - | - | - | 4.61 (± 0.17) × 107 | 155.95 ± 0.18 | 12.16 ± 0.23 |
| GBSN | α-amylase | 3.40 (± 0.15) × 105 | 2.38 (± 0.24) × 104 | 2.96 (± 0.25) × 104 | 35.21 ± 0.06 | 0.68 ± 0.08 |
| MBSN | α-amylase | - | - | 2.11 (± 0.16) × 104 | 32.34 ± 0.02 | 0.62 ± 0.11 |
| GBSN | β-amylase | - | - | |||
| + pullulanase | 1.27 (± 0.31) × 106 | 115.80 ± 0.12 | 0.82 ± 0.05 | |||
| MBSN | β-amylase | |||||
| + pullulanase | 4.02 (± 0.18) × 106 | 1.88 (± 0.29) × 105 | 5.22 (± 0.27) × 105 | 98.22 ± 0.10 | 0.55 ± 0.07 |
Data are expressed as the mean of three measurements ± SD.
The dispersed molecular densities defined as ρ = Mw/Rz3 (Liang et al., 2013) of GBS and MBS were also presented in Table 2. The ρ value of GBS was higher than that of MBS, suggesting that GBS might have greater branched and denser structure than MBS (Yoo and Jane, 2002). The denser structure implied that the starch particles need more energy to be destroyed, thus it might be difficult to gelatinize GBS.
Starch granule morphology SEMs of GBS and MBS were shown in Fig. 1. The GBS granules were small (particle size, 5∼18 µm), spherical or elliptic in shape with smooth surfaces. However, the particle size of MBS was relatively larger, ranging from 15 to 25 µm. It was generally accepted that smaller granules were in favor of making higher quality starch noodles because of the better freeze thaw stability of starch and the better fluidity of starch dough (Miao et al., 2012; Chen et al., 2003; Liu and Shen, 2007).
Morphology of starch granules by scanning electron microscopy (1000 ×): (a) GBS; (b) MBS
Gelatinization properties The To, Tp, Tc and ΔH values of GBS and MBS were summarized in Table 3. The DSC thermogram of GBS displayed a single peak at 73.12∼77.93∼84.97°C (To∼Tp∼Tc) with ΔH 9.83 kJ/kg, which were consistent to the research of Miao et al. (2012). MBS gave a single but broad peak with transition temperatures at 58.01∼66.06∼79.20°C that were lower than those of GBS. One probable explanation was that GBS had a more perfect crystal structure than MBS (Ratnayake et al., 2001). This result was consistent with the analysis given by the molecular distribution analysis. However, the ΔH value of MBS was higher than that of GBS. Tester and Morrison (1990) postulated that ΔH reflects the overall crystallinity (quality and amount of crystallites) of amylopectin. The higher ΔH of MBS suggested that disruption of crystallinity of amylopectin during gelatinization was more pronounced than GBS.
| Sample | Enzyme | To/°C | Tp/°C | Tc/°C | ΔH/(kJ. kg−1) |
|---|---|---|---|---|---|
| GBS | - | 73.12 ± 0.30 | 77.93 ± 0.43 | 84.97 ± 0.24 | 9.83 ± 0.15 |
| MBS | - | 58.01 ± 0.21 | 66.06 ± 0.36 | 79.20 ± 0.52 | 11.56 ± 0.28 |
| GBSN | - | 55.88 ± 0.27 | 60.33 ± 0.34 | 70.94 ± 0.54 | 1.99 ± 0.19 |
| GBSN | - | 102.25 ± 0.29 | 107.55 ± 0.37 | 113.41 ± 0.36 | 0.85 ± 0.16 |
| MBSN | - | 42.93 ± 0.32 | 52.05 ± 0.24 | 63.19 ± 0.44 | 2.29 ± 0.15 |
| MBSN | - | 100.93 ± 0.34 | 104.86 ± 0.41 | 111.25 ± 0.39 | 0.62 ± 0.18 |
| GBSN | α-amylase | 89.52 ± 0.36 | 98.69 ± 0.41 | 102.19 ± 0.34 | 0.64 ± 0.14 |
| MBSN | α-amylase | 84.82 ± 0.38 | 95.19 ± 0.49 | 100.55 ± 0.64 | 0.39 ± 0.10 |
| GBSN | β-amylase | 92.75 ± 0.44 | 100.59 ± 0.55 | 109.68 ± 0.54 | 4.81 ± 0.23 |
| + pullulanase | |||||
| MBSN | β-amylase | 88.93 ± 0.51 | 98.60 ± 0.42 | 109.58 ± 0.53 | 3.72 ± 0.21 |
| + pullulanase |
Data are expressed as the mean of three measurements ± SD.
Quality evaluation Table 4 compared the cooking quality, broken rate and textural properties of the two starch noodles prepared in laboratory, and two commercial products. The cooking loss is an indication of the cooking qualities of noodles and regarded as a resistance of noodles to the disintegration upon prolonged boiling (Tan et al., 2009). The Chinese Agriculture Trade Standards set a cooking loss less than 10% during cooking as accepted value and the cooking loss of GBSN was only 4% in the present study. GBSN exhibited lower swelling index than MBSN and c-MBSN but higher than c-SPSN. It was suggested that the water-holding capacity of GBSN was lower than MBSN and c-MBSN but was comparable with commercial noodles. The most important weakness of GBSN was its broken rate which was much higher than that of MBSN or commercial products.
| Samples | Cooking loss/% | Swelling index /% | BR/% | Hardness/g | Adhesiveness/(g·s) | Springiness | Cohesiveness |
|---|---|---|---|---|---|---|---|
| GBSN | 4.02 ± 0.18b | 593.17 ± 15.41b | 100 ± 6a | 476.81 ± 18.41d | 9.19 ± 1.21b | 0.78 ± 0.11a | 0.39 ± 0.08a |
| MBSN | 3.78 ± 0.15b | 635.62 ± 18.32a | 20 ± 4c | 634.17 ± 20.61c | 2.32 ± 0.43c | 0.84 ± 0.16a | 0.40 ± 0.13a |
| c-MBSN | 2.80 ± 0.11c | 661.36 ± 19.12a | 10 ± 2d | 1380.39 ± 39.22a | 0.30 ± 0.13d | 0.96 ± 0.18a | 0.44 ± 0.11a |
| c-SPSN | 8.23 ± 0.28a | 462.97 ± 13.52c | 30 ± 2b | 1124.23 ± 51.32b | 13.43 ± 2.14a | 0.93 ± 0.13a | 0.43 ± 0.13a |
Data are expressed as the means of three measurements ± SD and the mesns followed by the same letter within the same column are not significantly different at p <0. 05 by LSD test compared to GBSN.
According to Buddhagal and Noomhorm (2002), good-quality starch noodles should have firm texture, low stickness, high springiness and cohesiveness. Compared to MBSN, c-MBSN and c-SPSN, GBSN possessed the lowest hardness. As to the springiness and cohesiveness, GBSN showed no obvious difference from MBSN, c-MBSN and c-SPSN, indicating that some of textural properties of GBSN were close to those of commercial products but should still be further improved.
Microscopic structure of GBSN and MBSN Fig. 2 showed SEMs on the surface and transverse section of uncooked GBSN and MBSN. In the micrographs, the outline and identity of starch granules were not observed, probably due to the complete gelatinization during the noodles preparation process resulting in the amorphous matrix formation (Jyotsna et al., 2004). Under the same drying conditions, MBSN had a smoother surface than GBSN, which might be due to its stronger gel strength and elasticity that withstood better the shrinkage (Tan et al., 2006). The unstable press during the starch dough making and water release during cooling led to the formation of air holes inside the starch noodles, which could influence the qualities of starch noodles.
Scanning electron micrographs of both starch noodles: (a) the surface of GBSN (1000×); (b) the transverse section of GBSN (150×); (c) the surface of MBSN; (d) the transverse section of MBSN
Enzymatic digestion of GBSN and MBSN The enzymatic hydrolysis is a digestion simulation process of starch noodles in the human body, but the starch digestion in vitro might be slower than digestion in vivo (about 1.5∼3 h) (Heaton et al. 1988). The digestion was conducted with two enzyme systems: α-amylase and β-amylase + pullulanase. The hydrolysis was fast during the first 36 h followed by a slow rate between 36 h and 120 h, which were observed in Fig. 3, therefore the cooked GBSN and MBSN treated by α-amylase showed a similar two-stage hydrolysis pattern. The hydrolysis phenomenon resembled with the result of Tan et al. (2006), but the hydrolysis efficiency was improved and equilibrium was achieved faster. The first stage of hydrolysis corresponded to the degradation of the more amorphous region of the noodles, leading to a hydrolysis degree of approximately 80%. During the second stage, the more compact parts were slowly degraded. The final maximum hydrolysis degree of GBSN and MBSN was 85.2% and 86.5% respectively.
Hydrolysis of cooked GBSN and MBSN using α-amylase at 35°C for 120 h (a) and β-amylase + pullulanase at 35°C for 60 h (b)
Digestion of cooked GBSN and MBSN with β-amylase + pullulanase displayed a similar kinetics curve but the period of fast hydrolysis was shortened to 24 h approximately. The final hydrolysis degree of GBSN and MBSN was 84.1% and 85.3% respectively. The hydrolysis pattern of GBSN and MBSN digested by α-amylase or β-amylase + pullulanase was similar but the combination of the two enzymes seemed to be more efficient than α-amylase alone.
Molecular weight distributions of enzymatic digestion residues The Mw, Rz and ρ values of cooked GBSN and MBSN residues resistant to α-amylase or β-amylase + pullulanase were also summarized in Table 2. Two peaks were observed in the α-amylase residue of GBSN. The Mw of peak 1 and peak 2 were 3.40 × 105 and 2.38 × 104 kg/kmol respectively, and the average Mw was 2.96 × 104 kg/kmol. However, only one peak was detected for the α-amylase residue of MBSN and the Mw was 2.11 × 104 kg/kmol. It was noted that the Mw of residues of both starch noodles were much lower than the original starch, indicating that the long chains in starches were degraded to short chains. The Mw of GBS was lower than MBS in the above starch analysis, whereas GBSN displayed higher Mw than MBSN after α-amylase hydrolysis. It implied that the degradation of MBSN was more complete than GBSN. The Mw values of GBSN residues suggested that it was more resistant to α-amylase than MBSN residues. The variation of Rz for both residues altered with Mw, as shown in Table 2.
The residue of GBSN resistant to the combined action of β-amylase + pullulanase showed only one peak, and the Mw was 1.27 × 106 kg/kmol. Two peaks of MBSN residue were observed, and the obtained average Mw was 5.22 × 105 kg/kmol. The Mw value of GBSN residue treated by β-amylase + pullulanase was relatively uniform and higher than that of MBSN residue, which revealed that MBSN was degraded more completely than GBSN during hydrolysis by β-amylase + pullulanase. The ρ value of GBSN residue was higher than that of MBSN residue, suggesting that the GBSN residue might have more densely packed structure. After two types of enzymes hydrolysis, GBSN remained larger fractions than MBSN, indicating that GBSN was more resistant to enzyme digestion. The phenomenon was consistent with enzymatic hydrolysis test.
Their behaviors in the enzymatic hydrolysis revealed that the starch in noodles was composed of two parts: one part had relatively loose structure and could be easily degraded, and the other part possessed compact structure and its degradation was much slower. Miao et al. (2012) have demonstrated that ginkgo starch was notably resistant than cereal starches because of its higher amylose content. The compact part of starch noodles was resistant to amylase digestion, which was just like slowly digestible starch that has potentially healthy benefits to human body.
Thermal properties The thermal properties of GBSN, MBSN and their residues resistant to enzymes were given in Table 3. The DSC thermogram of GBSN between 30°C and 120°C showed two peaks at 55.88∼60.33∼70.94°C (To∼Tp∼Tc) with ΔH 1.99 kJ/kg, and 102.25∼107.55∼113.41°C with ΔH 0.85 kJ/kg. The first melting peak was corresponding to the recrystallization of amylopection, and the first melting peak was the recrystallization of amylose or the complex of amylose and lipid (Jeong and Lim, 2003). The first peak melting temperatures of both starch noodles were lower than those of the original starch. The result of this phenomenon could be that the recombined molecules of gelatinized starch were not sufficiently stable and ordered as the original starch molecules, so the melting temperatures of the recombined starch were lower (Tan et al., 2006). The thermal properties changing phenomenon of MBSN was similar to GBSN, as reported by Kim et al. (2007).
The GBSN and MBSN residues digested with α-amylase displayed two peaks on DSC thermogram at 89.52∼98.69∼102.19°C with ΔH 0.64 kJ/kg and 84.82∼95.19∼100.55°C with ΔH 0.39 kJ/kg, respectively. The GBSN residue resistant to β-amylase + pullulanase showed higher melting temperatures and higher ΔH than those of MBSN. It was probably attributed to the more compact molecular structure of GBSN residue than MBSN, which caused difficulty for enzymes to reach the inner structure of GBSN residue. The gelatinization temperatures and enthalpies of both starch noodles residues of β-amylase + pullulanase digestion were higher than those of residues resistant to α-amylase, suggesting that both starch noodles were more resistant to β-amylase + pullulanase. This observation might be due to the fact that starch noodles digested by β-amylase + pullulanase could release more B-chains that functioned as the short amylose molecules and formed double helical complex crystal itself and helical complex with lipid (Chung et al., 2003).
Ginkgo and mung bean starch contained similar amylose content, and the comparative results of physicochemical properties between both starches indicated that ginkgo starch might be a suitable material for producing starch noodles. Ginkgo starch had denser structure, smaller particle size and higher gelatinization temperatures than mung bean starch. The quality evaluation of starch noodles showed that ginkgo starch noodles had comparable quality with mung bean and commercial starch noodles concerning cooking loss, swelling index, springiness and cohesiveness, but has lower broken rate, hardness and adhesiveness quality, which should be improved. After enzymatic digestion of starch, ginkgo starch noodles displayed a little lower hydrolysis degree and larger fractions than mung bean starch noodles, indicating that ginkgo starch noodles were more resistant to enzyme actions. The thermal properties showed that both ginkgo and mung bean starch noodles were more resistant to β-amylase + pullulanase than α-amylase. Enzymatic digestion analysis results suggested that two parts of starch in ginkgo and mung bean starch noodles were distinguished: one part could be easily degraded by α-amylase or β-amylase + pullulanase, and the other would be more resistant to enzyme actions. The property of the second part of starch in ginkgo starch noodles might be more like “slowly digestible starch” which is considered as beneficial to human health.
